Gene transfer to pulmonary epithelia is a direct approach to correct the ion transport and host defense defects associated with cystic fibrosis (CF). Unfortunately, several problems currently preclude its clinical application including poor apical gene transfer efficiency, transient expression, adverse immune responses, and a limited understanding of the intracellular fate of integrating vectors. Our recent work funded through this program focused on the development and application of recombinant lentiviral vectors based on feline immunodeficiency virus (FIV) to surmount these problems. Initial steps in retroviral transduction require interactions between the viral envelope glycoprotein (GP) and receptors on the host cell. For many vector envelopes, the cellular receptors are functionally absent from the apical surface of airway epithelia. Significant progress has come through our detailed studies of the interactions between the airway cell and envelope GPs. Recently we successfully pseudotyped FIV using the GPs from several enveloped viruses to direct vector binding and entry to the apical surface of airway epithelia. Importantly, FIV vector pseudotyped with envelope glycoproteins from the arenavirus lymphocytic choriomeningitis virus (LCMV) strain WE54 enters polarized human airway epithelia from the apical surface without the disruption of tight junctions. Therefore we now have a useful reagent to address important issues regarding the interactions between an integrating vector and airway epithelia. Our central hypothesis is that LCMV pseudotyped FIV can enter airway epithelia via the apical surface, allowing for persistent correction of the CF defect by gene transfer. We will focus on four main questions: 1) What is the receptor for LCMV pseudotyped FIV in airway epithelia? 2) Are there apical barriers to gene transfer with LCMV pseudotyped FIV in airway epithelia? 3) Does LCMV pseudotyped FIV confer persistent transgene expression in human airway epithelia? 4) If FIV enters airway epithelia through the apical surface in vivo will it result in persistent transgene expression? Completing these aims will address key questions regarding FIV-airway cell interactions and improve our ability to apply lentiviral vectors for the treatment and prevention of cystic fibrosis lung disease.